fix(chatgpt): handle truncated annotation responses

[H-TTTTT]

Summary

• Validate OpenAI-compatible annotation responses and log bounded diagnostics without provider payloads.
• Retry token-truncated segments once after UTF-8-safe splitting and save only complete results.
• Harden annotation admin/rendering data handling and inline JSON output.

Closes #1412

Testing

• php inc/chatgpt/annotation-response.test.php (29 tests, 0 failures)
• PHP syntax checks for all changed PHP files
• git diff --check

Risks

• No live provider or WordPress integration testing was performed; provider-specific response variants and admin UI behavior remain integration risks.